Zirconia refractories for making steel

ABSTRACT

A refractory material for steelmaking is comprised of 50 to 95% zirconia (ZrO 2 ), 0 to 35% of silica (SiO 2 ), 5 to 35% carbon (C), less than 5% alumina (Al 2 O 3 ) and anti-oxidant such as silicon (Si) metal and a carbide. The anti-oxidant may be in amounts up to about 10% by weight. The refractory material has application in making delivery nozzles and transition pieces. Disclosed also is a method of continuous casting steel strip comprising the steps of assembling a pair of casting rolls having a nip between them; assembling a delivery system for delivering molten steel to form a casting pool comprised of at least some refractory material for contacting the molten steel comprised of 50 to 95% zirconia, 0 to 35% silica, 5 to 35% carbon, less than 5% alumina and anti-oxidant and carbides, and rotating the casting rolls to form thin steel strip delivered downwardly through the nip.

BACKGROUND AND SUMMARY

This invention relates to making of steel, and more particularly tomaking of steel by continuous casting. The invention has application inmaking steel by casting steel strip.

In continuous casting of thin strip steel, molten steel is delivered toa caster from a ladle through a tundish and melt delivery systemincluding a shroud, a delivery nozzle and sometimes a transition piece.These metal delivery components deliver molten steel at 1500 to 1600°C., or more, to the caster through refractories which are capable ofwithstanding the high temperatures in the process of casting moltensteels. These refractories may also be preheated to the deliverytemperature to avoid thermal shock when the molten steel is introducedthrough the delivery system. These metal delivery components are used incontinuous casting by, among others, thick slab casters, thin slabcasters and thin strip casters. Illustrative of the refractory materialsfor such metal delivery components particularly useful in thin stripcasting are those described in U.S. Pat. Nos. 5,924,476 and 6,257,315.

In thin strip casting, molten metal is typically introduced between apair of counter rotated horizontally positioned casting rolls to form anip between them. The casting rolls are internally cooled so that metalshells solidify on the moving roll surfaces and are brought together atthe nip to produce a cast strip delivered downwardly from the nip. Theterm “nip” is used herein to refer to the general region at which thecasting rolls are closest together. The molten metal may be poured froma ladle into a smaller vessel from which the metal flows through adelivery nozzle located above the nip to form a casting pool supportedon the casting surfaces. The casting pool is supported on the castingrolls adjacent the nip and extending along the length of the nip. Thecasting pool is usually confined between side plates or dams held insliding engagement with end surfaces of the casting rolls so as to damthe two ends of the casting pool against outflow.

A problem in such metal delivery systems is that the molten slag tendsto stick to the refractories of the delivery system. This has been knownto occur even when the refractories are pre-heated to the temperature ofthe molten steel. The buildup of slag that occurs on the refractoriesfrom the molten steel has a tendency to break away, and cause defects toform in the cast steel and on the surface of the cast steel. This isparticularly true of the build up that occurs at the meniscus of thecasting pool in thin strip casters.

We have found a refractory of particular composition that inhibits thebuildup of the molten steel on refractories of a metal delivery systemin continuous casting of steel. The refractory material for such thinstrip steelmaking is comprised of 50 to 85% zirconia (ZrO₂), 0 to 35% ofsilica (SiO₂), 5 to 35% carbon (C), less than 5% alumina (Al₂O₃) and ananti-oxidant.¹ The purity of the carbon used may be greater than 99.5%.The refractory material may be stabilized with lime or magnesium oxide,and typically in an amount less than about 28%. The refractory materialhas application in making delivery nozzles and transition piece nozzleblocks for use in making steel by continuous casting of steel strip. Therefractory material may by used, for example, in making a deliverynozzle for use in making steel by continuous casting of steel strip.1 All percentages are stated percent by weight.

Also disclosed is a method of continuous casting of steel stripcomprising the steps of:

-   -   a. assembling a pair of cooled casting rolls having a nip        between them and with confining closure adjacent the ends of the        nip;    -   b. assembling a metal delivery system for delivering a molten        steel between the casting rolls to form a casting pool supported        therebetween comprised of at least some refractory material for        contacting the molten steel, the refractory material comprised        of 50 to 85% zirconia, 0 to 35% silica, 5 to 35% carbon, less        than 5% alumina and an anti-oxidant, and    -   c. counter rotating the casting rolls to form metal shells on        the surfaces of the casting rolls and solidified thin steel        strip delivered downwardly through the nip between the casting        rolls.

The refractory material may have a zirconia (ZrO₂) content between 60and 85%, and more specifically between 70 and 80% by weight. Therefractory material may have a carbon content between 8 and 30%, andmore specifically between 10 and 20% by weight. The zirconia may bestabilized or unstabilized, but again the refractory material may bestabilized with lime or magnesium oxide or a combination thereof toreduce wear on the refractory material during use.

The anti-oxidant inhibits oxidation of the other components of therefractory material, and may be any one or a combination of suchmaterials that inhibits oxidation in the refractory material system. Theanti-oxidant may be present in an amount up to about 10% by weight.Examples of antioxidants are, without limitation, Si metal, Al metal,silicon aluminum alloy and carbides such as boron carbide and siliconcarbide.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully explained one particularembodiment is described with respect to continuously casting of steelstrip and with reference to the accompanying drawing in which:

FIG. 1 illustrates the flow of molten steel through a metal deliverysystem into a twin-roll caster.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A twin-roll caster 11 comprises a main machine frame 21 which supports apair of internally cooled casting rolls 22 having casting surfaces 22A.The casting rolls 22 are positioned laterally adjacent each other toform a nip 27 between them and through which cast steel strip may beformed. The twin-roll caster may be as illustrated in U.S. Pat. Nos.5,184,668, 5,277,243 and 5,488,988, to which reference can be made formore detail.

Molten metal is supplied for the continuous casting of strip from theladle (not shown) to a tundish 23. From the tundish 23 the molten metalis delivered through the metal delivery system by refractory linedshroud 24 to a transition piece nozzle block 25, which is alsorefractory lined. Stopper 18 is seated into refractory inlet 21 andattached to shroud 24 at connection 17 to regulate the flow of moltenmetal from the tundish 23 into the shroud 24. Stopper 18 is moveable toregulate the flow of molten metal from the tundish 23 into shroud 24.

Transition piece nozzle block 25 is configured to generally enclose themolten metal from exposure to the outside atmosphere, with an overflow19 through which molten metal can flow should the metal in thetransition piece reach a point of overflowing. The molten metal isdelivered from the shroud 24 into transition piece 25 usually below thefill-line 16 of the molten metal in the transition piece to minimizeexposure of the molten metal to air.

From the transition piece nozzle block 25, the molten metal is deliveredto the casting pool 30 through a delivery nozzle 26 made of refractorymaterial. The upper surface 31 of the casting pool 30 (generallyreferred to as the meniscus level) may rise above the lower end of thedelivery nozzle 26 so that the lower end of the delivery nozzle isimmersed within the casting pool 30, which is confined at the ends ofthe rolls by a pair of side closure dams or plates 28.

Casting pool 30 is positioned above the nip 27 between the casting rolls22 supported on the casting roll surfaces 22A. The casting rolls 22 aredriven to counter-rotate, and the casting rolls 22 are cooledinternally, usually with circulation of water. As the casting rollsrotate, shells of metal solidify from the casting pool 30 on the movingcasting roll surfaces 22A. The shells are in turn brought together atthe nip 27 between casting rolls 22 to produce solidified strip 12delivered downwardly from the nip 27.

The transition piece nozzle block 25 and the delivery nozzle 26 may bemade of the refractory material of the present invention. The refractorymaterial is comprised of 50 to 85% by weight zirconia, 0 to 35% byweight of silica, less than 5% by weight alumina, 5 to 35% by weightcarbon and an anti-oxidant. The refractory material may have a zirconia(ZrO₂) content between 60 and 85%, and more specifically between 70 and80% by weight. The refractory material may have a carbon content between8 and 30%, and more specifically between 10 and 20% by weight. Theanti-oxidant may be up to about 10% by weight, and may be, for example,Si metal, Al metal, silicon aluminum alloy or a carbide such as boroncarbide or silicon carbide. The refractory may be stabilized orunstabilized, but it may be stabilized with lime (CaO)Or magnesium oxide(MgO), or a combination thereof, to reduce wear on the refractorymaterial during use in contact with molten steel. The amount of lime ormagnesium oxide may be in an amount less than about 28% by weight.

An example of such refractory material may have the followingcomposition: Zirconia 74% Silica 06% Carbon 12%

The remainder of the chemical composition of the refractory may be othermaterials such as lime (e.g. 3%) which are purposefully added forstabilization, and impurities. The refractory in any case is a carbonbonded silica graphite.

Typical physical properties of the refractory of the specificcomposition are as follows: Bulk density 3.70 g/cc Apparent Porosity 15%Modulus Rupture (rt) 1000 psi

The advantage of the refractory as described is that molten metal doesnot stick to the refractory and form slag as the molten metal flowsthrough in contact with the refractory. Such slag usually collects atthe meniscus of the casting pool 30, from where the slag breaks off andgoes into the shells formed during solidification and into the strip 12to produce defects in the strip and surface defects in the strip. Withthe zirconia carbon refractories of the present invention, such slagformation is inhibited and strip quality is improved in continuouslymaking strip by the twin-roll caster.

While the invention has been illustrated and described in detail in theforegoing drawings and description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly illustrative embodiments thereof have been shown and described andthat all changes and modifications that come within the spirit of theinvention are desired to be protected. Additional features of theinvention will become apparent to those skilled in the art uponconsideration of the description. Modifications may be made withoutdeparting from the spirit and scope of the invention.

1. A refractory for molten metal delivery in making steel comprising 50to 85% by weight zirconia, 0 to 35% by weight silica, less than 5%alumina, 5 to 35% carbon and an anti-oxidant.
 2. The refractory formolten metal delivery in making steel as claimed in claim 1 wherein thepercent by weight of zirconia is between 60 and 85% by weight.
 3. Therefractory for molten metal delivery in making steel as claimed in claim1 wherein the percent by weight of zirconia is between 70 and 80% byweight.
 4. The refractory for molten metal delivery in making steel asclaimed in claim 1 wherein the percent by weight of carbon is between 8and 30% by weight.
 5. The refractory for molten metal delivery in makingsteel as claimed in claim 1 wherein the percent by weight of carbon isbetween 10 and 20% by weight.
 6. The refractory for molten metaldelivery in making steel as claimed in claim 1 wherein the refractorymaterial is stabilized with lime, magnesium oxide or a combinationthereof.
 7. The refractory for molten metal delivery in making steel asclaimed in claim 1 wherein the anti-oxidant comprises up to about 10% byweight.
 8. A refractory for molten metal delivery in making steel bycontinuous casting steel strip comprising 50 to 85% zirconia, 0 to 35%silica, less than 5% alumina, 5 to 35% carbon and an anti-oxidant. 9.The refractory for molten metal delivery in making steel by continuouscasting strip as claimed in claim 8 wherein the percent by weight ofzirconia is between 60 and 85% by weight.
 10. The refractory for moltenmetal delivery in making steel by continuous casting strip as claimed inclaim 8 wherein the percent by weight of zirconia is between 70 and 80%by weight.
 11. The refractory for molten metal delivery in making steelby continuous casting strip as claimed in claim 8 wherein the percent byweight of carbon is between 8 and 30% by weight.
 12. The refractory formolten metal delivery in making steel by continuous casting strip asclaimed in claim 8 wherein the percent by weight of carbon is between 10and 20% by weight.
 13. The refractory for molten metal delivery inmaking steel by continuous casting strip as claimed in claim 8 whereinthe refractory material is stabilized with lime, magnesium oxide or acombination thereof.
 14. The refractory for molten metal delivery inmaking steel by continuous casting strip as claimed in claim 8 whereinthe anti-oxidant comprises up to about 10% by weight.
 15. A deliverynozzle for making steel by continuously casting steel strip having acomposition comprising 50 to 85% zirconia, 0 to 35% silica, less than 5%alumina, 5 to 35% carbon, and an anti-oxidant.
 16. The delivery nozzlefor making steel by continuous casting strip as claimed in claim 15wherein the percent by weight of zirconia is between 60 and 85% byweight.
 17. The delivery nozzle for making steel by continuous castingstrip as claimed in claim 15 wherein the percent by weight of zirconiais between 70 and 80% by weight.
 18. The delivery nozzle for makingsteel by continuous casting strip as claimed in claim 15 wherein thepercent by weight of carbon is between 8 and 30% by weight.
 19. Thedelivery nozzle for making steel by continuous casting strip as claimedin claim 15 wherein the percent by weight of carbon is between 10 and20% by weight.
 20. The delivery nozzle for making steel by continuouscasting strip as claimed in claim 15 wherein the refractory material isstabilized with lime, magnesium oxide or a combination thereof.
 21. Thedelivery nozzle for making steel by continuous casting strip as claimedin claim 15 wherein the anti-oxidant comprises up to about 10% byweight.
 22. A transition piece nozzle block for flow control in makingsteel by continuously casting steel strip having a compositioncomprising 50 to 85% zirconia, 0 to 35% silica, less than 5% alumina, 5to 35% carbon and an antioxidant.
 23. The transition piece nozzle blockfor making steel by continuous casting strip as claimed in claim 22wherein the percent by weight of zirconia is between 60 and 85% byweight.
 24. The transition piece nozzle block for making steel bycontinuous casting strip as claimed in claim 22 wherein the percent byweight of zirconia is between 70 and 80% by weight.
 25. The transitionpiece nozzle block for making steel by continuous casting strip asclaimed in claim 22 wherein the percent by weight of carbon is between 8and 30% by weight.
 26. The transition piece nozzle block for makingsteel by continuous casting strip as claimed in claim 22 wherein thepercent by weight of carbon is between 10 and 20% by weight.
 27. Thetransition piece nozzle block for making steel by continuous castingstrip as claimed in claim 22 wherein the refractory material isstabilized with lime, magnesium oxide or a combination thereof.
 28. Thetransition piece nozzle block for making steel by continuous castingstrip as claimed in claim 22 wherein the anti-oxidant comprises up toabout 10% by weight.
 29. A method of continuously casting steel stripcomprising the steps of: a. assembling a pair of cooled casting rollshaving a nip between them and with confining closure adjacent the endsof the nip; b. assembling a metal delivery system for delivering amolten steel between the casting rolls to form a casting pool supportedtherebetween comprised of at least some refractory material forcontacting the molten steel comprised of 50 to 85% zirconia, 0 to 35%silica, less than 5% alumina, 5 to 35% carbon and anti-oxidant; and c.counter-rotating the casting rolls to form metal shells on the surfacesof the casting rolls and solidified thin steel strip delivereddownwardly through the nip between the casting rolls.
 30. The method ofcontinuously casting steel strip as claimed in claim 29 wherein thepercent by weight of zirconia is between 60 and 85% by weight.
 31. Themethod of continuously casting steel strip as claimed in claim 29wherein the percent by weight of zirconia is between 70 and 80% byweight.
 32. The method of continuously casting steel strip as claimed inclaim 29 wherein the percent by weight of carbon is between 8 and 30% byweight.
 33. The method of continuously casting steel strip as claimed inclaim 29 wherein the percent by weight of carbon is between 10 and 20%by weight.
 34. The method of continuously casting steel strip as claimedin claim 29 wherein the refractory material is stabilized with lime,magnesium oxide or a combination thereof.
 35. The method of continuouslycasting steel strip as claimed in claim 29 wherein the anti-oxidantcomprises up to about 10% by weight.